re PR c/24010 (Duplicate C99 dot initializer warning missing)

re PR c/24010 (Duplicate C99 dot initializer warning missing)

	PR c/24010
	* c.opt (Woverride-init): New.
	* c-opts.c (c_common_post_options): Default warn_override_init to
	extra_warnings.
	* c-typeck.c (add_pending_init, output_init_element): Warn for
	overriding initializers without side effects.
	* doc/invoke.texi (-Woverride-init): Document.
	(-Wextra): Update.

testsuite:
	* gcc.dg/Woverride-init-1.c, gcc.dg/Woverride-init-2.c,
	gcc.dg/Woverride-init-3.c: New tests.

From-SVN: r117334